1. Field of the Invention
The present invention relates to a method of manufacturing multilayer-type inductor and, more particularly, to multilayer-type inductors having small tolerances for inductance values while retaining the ability to provide many different inductance values using one set of component parts, and to said multilayer inductor.
2. Description of the Related Art
Conventional multilayer-type inductors are disclosed in Japanese Examined Patent Publication No. 57-39521, and in Japanese Unexamined Utility Model Publication No. 57-100209. The inductor of the former publication is manufactured in the following way. A first coil conductor annularly extending about 90.degree., i.e., for about a quarter turn, is formed on a thin plate of magnetic material, and a layer of magnetic material is formed on this first coil conductor. Thereafter, a second coil conductor extending for about a quarter turn is formed on the magnetic layer so that one end of the second coil conductor is connected to the first coil conductor. Next, another magnetic layer is formed on the second coil conductor with the other end of the second coil conductor being exposed and, subsequently, the same process is predetermined number of times so that the quarter turn coil conductors are stacked in layers in the direction of multilayer, i.e., normal to the plane of the thin plate.
The inductor as disclosed in the later publication is manufactured in the following way. A hole is provided in a part of a magnetic sheet. One end of a U-shaped coil conductor is positioned at this hole so that a part of the coil conductor is exposed at the rear surface of the sheet through the hole. By stacking these sheets so that the open end of U-shaped coil conductors face each other but in different planes, the coil conductors of each sheet are connected to each other to form a coil structure.
Generally, if the inductance value of the inductor is denoted as L, the specific magnetic permeability of a material used for an insulating layer as .mu..sub.s, the inner area of a coil as S, and the length of the coil as l, the following equation (1) is satisfied: EQU L=.mu..sub.s .mu..sub.o kS/l (1)
where k is a constant proportional to the square of the number of windings of the coil, and .mu..sub.o is the magnetic permeability of the material used for the insulating layer.
Therefore, in both inductors described above, in order to obtain many different inductance values, the number of windings of the coil, the diameter of the coil (i.e., the inner area of the coil), the specific magnetic permeability of the material used for the insulating layer, and the thickness of the insulating layer may be appropriately set. However, in order to set a very small inductance value, changing the specific magnetic permeability, the thickness of the insulating layer, and the number of windings of the coil is not suitable, and normally, a predetermined inductance value is obtained by adjusting the diameter of the coil by varying the pattern of the coil conductor.
In this case, it is necessary to prepare a pattern of the coil conductor for each inductance value. The manufacturing cost of the pattern and the cost for changing the stage for the pattern during manufacturing are large, hindering a reduction in the product price. As a countermeasure to this, a method has been proposed in which the inductance value is adjusted by replacing some layers of coil conductors to be stacked with coil conductors having a smaller diameter. However, this method is used only in some inductors because the management of the multilayering process becomes complex and the frequency characteristic deteriorates due to the disturbances of the magnetic flux.